Investigation of a Novel Method to Quantify Respiratory-Gated Beam-Latency on Truebeam Linear Accelerators
Abstract
Purpose
To demonstrate a novel method of quantifying respiratory gating beam-on and beam-off latency for Varian TrueBeam linear accelerators and assess the impact of dose rate and machine-specific factors on overall timing performance.
Methods
Latency measurements were performed using a modified QUASAR Respiratory Motion Phantom (pRESP) (IBA QUASAR, London, Canada) driven by sinusoidal motion (20mm peak-to-peak and 15 breaths per minute), synchronized with the Varian respiratory motion infrared camera gating system. A proprietary pulse conditioning circuit receives a target pulse signal from the TrueBeam system and conditions that signal into a TTL output. The TTL output then uses logic high to represent Beam On and logic low to represent Beam Off. The TTL signal is fed into the motion phantom input to allow for beam on/off latency measurements. Five TrueBeam units at our centre were evaluated using open-field respiratory-gated 6FFF beam with a 50% gating window and varying dose rates.
Results
The mean beam-on latency across machines ranged from approximately 35–55 ms, with modest variation between units over the investigated dose-rate range. Mean beam-off latency was larger, spanning roughly 65–95 ms across the same machines and dose rates, but remained below the 100 ms latency consensus criterion recommended by the AAPM.
Conclusion
The custom measurement setup enabled robust characterization of TrueBeam respiratory gating latencies under clinically relevant conditions. Across multiple machines and dose rates, both beam-on and beam-off latencies met the 100 ms tolerance, supporting the clinical feasibility of gated delivery with 6FFF beams on TrueBeam systems. The methodology and updated hardware provide a framework for routine latency verification and future multi-institutional comparisons.